1. Field of the Invention
The present invention generally relates to a passive infrared detection system and, more particularly, is concerned with a cross-over field-of-view composite Fresnel lens for an infrared detection system.
2. Description of the Prior Art
A passive infrared detection system detects heat energy radiated or emitted by an object, such as the body of a person, moving across the field of view of a heat sensor, such as a pyroelectric detector, of the detection system. The detection system must be capable of distinguishing between background temperature and the moving object having a different temperature from the background. Thus, the detection system has an electrical circuit operatively coupled to the heat sensor for producing a detection signal in response to the heat sensor detecting a change of temperature, as for example, caused by the body heat of a person entering the detection pattern.
The sensitive area of the heat sensor is too small to detect a significant amount of heat energy radiated from a human body by using the heat sensor alone. Thus, detection systems typically employ various mirror and/or Fresnel lens arrangements for collecting and focusing a significantly greater amount of heat energy on the heat sensor than would be received using the heat sensor alone. Also, the optics allows the sensor to look in many directions simultaneously.
Many different mirror and/or Fresnel lens arrangements employed in conjunction with heat sensors are known in the prior patent art. A Fresnel lens is a lens resembling a plano-convex or plano-concave lens. One of the opposite sides of the lens is smooth whereas the other side is cut into narrow rings forming annular grooves thereon. Fresnel lens are conventionally arranged either with the infinite or longer conjugate on the grooved side of the lens which is a constructional characteristic commonly referred to as "grooves out" or with the infinite or longer conjugate on the smooth side of the lens which is a constructional characteristic commonly referred to as "grooves in".
Representative examples of mirror and Fresnel lens arrangements are disclosed in U.S. Pat. Nos. to Wagli et al (4,429,224) and Lederer (4,442,359) and of Fresnel lens arrangements alone are disclosed in U.S. patents to Mudge (4,275,303), Galvin et al (4,321,594), Vigurs (4,535,240), Messiou (4,717,821) and Biersdorff (4,821,284) and in a U.K. patent application to Bioice (GB 2,112,339). A significant advantage of a Fresnel lens arrangement alone over a combination mirror and Fresnel lens arrangement is the reduction in the complexity of the arrangement.
Because the combination of a heat sensor with a Fresnel lens arrangement appears to be the more cost-effective approach, but will still constitute a significant portion of the cost of the detection system, the provision of a Fresnel lens arrangement which only requires the use of one heat sensor containing an array of elements and, at the same time, is highly effective and efficient in collecting and focusing heat energy on that heat sensor remains an important objective. The above-cited U.S. patents to Galvin et al and Messiou have proposed different Fresnel lens arrangements which are molded onto either flat or curved sheets of a suitable plastic, such as polyethylene, being transparent to infrared energy within the spectrum of interest while being translucent to visible light. It is perceived by the inventor herein that while the proposed arrangements of the above-cited U.S. patents to Galvin et al and Messiou may represent steps in the right direction, neither has come close to the attainment of the aforementioned objective. Thus, a pressing need still exists to design a Fresnel lens arrangement for use in a passive infrared detection system which will more closely reach the aforementioned objective of cost-effectiveness as well as achieve a 180.degree. field of view.